What is sea ice?
Below-freezing temperatures in the Arctic and Antarctic cause the ocean’s surface to freeze into ice crystals. When the water is calm, those crystals form wafer-thin sheets of ultra-smooth ice that slide over each other and freeze together into thick rafts. In choppy waters, ice crystals form slushy pancake-like chunks that can merge to create rough ice. Ridges form where wave action causes the pancakes to collide. Wind, waves, and currents move loose pieces of ice, called drift ice, across the water, creating openings where new ice can form or pile ice pancakes together. Some freezes to the coastline or ocean floor where it remains in place. Such ice is called fast ice.
Sea ice grows over the winter and recedes in summer. The thicker the ice, the more likely it is to last through the summer melt, leading to still thicker layers of sea ice the following year. Such multiyear ice hardens each year, leading to overall gains in local ice cover. First-year ice, on the other hand, tends to be thin and is more likely to melt during the summer.
Sea ice creates a barrier between the ocean and atmosphere, reducing movement of heat between the two environments. As the ice layer grows thicker, less heat is able to move across it. Once the ice reaches 3 to 4 meters (10 to 13 feet) thick, it effectively shuts down interactions between the air and water.
Why is it important?
As seawater freezes, ice crystals expel salt, which creates freshwater ice that floats on the ocean’s surface and releases salt into the water below. The increase in salinity creates dense water that sinks to the ocean floor, where it begins a slow migration toward the equator, contributing to the global ocean conveyor belt, the Atlantic Meridional Overturning Circulation (AMOC). This massive current redistributes energy throughout the world’s oceans and contributes to global climate.
The surface of the ice plays a critical role in reducing incoming energy from sunlight. Ice is highly reflective; most incoming sunlight bounces off the surface and returns to space. This keeps temperatures near the poles relatively cool, even during summer months. As ice begins to melt, however, it forms pools of dark water that absorb sunlight. This warms the water, further melting the ice and creating larger pools of water that absorb more sunlight. This feedback loop can contribute to rapid collapses of sea ice, making polar regions especially vulnerable to climate change.
Summer melt along the sea ice edge releases trapped nutrients into the water, which leads to increases in phytoplankton—tiny photosynthetic organisms that form the base of the marine food web. This attracts a variety of marine life, including higher-level predators, such as fish and whales, to areas along the ice rim. The ice itself provides essential habitat for marine mammals, including polar bears, seals, and Arctic foxes. These animals use the ice for birthing, hunting, and nursing their young. As sea ice declines, polar bears, in particular, are struggling to survive, and in their efforts to find food, they are encountering humans more frequently.
Because sea ice is created from seawater, it does not contribute to rising sea levels when it melts.
How are scientists studying sea ice?
Since 1979, satellites have allowed for continuous measurements of sea ice extent—the total area covered by ice—as well as its overall depth. This has allowed researchers to track sea ice as it expands in winter and retreats in summer. These records show that sea ice is now covering less area in both summer and winter in most years, with the Arctic most strongly affected by changes in global climate. In the last two decades, summertime losses have exceeded those seen prior to the 1980s and 1990s, and the most extreme ice loss events have happened in the last 15 years. Models predict that by the year 2100, the Arctic will be completely free of ice in summer.
Scientists also rely on historical records, such as those kept by Vikings and European sailors that made note of Arctic ice at various locations and times. They also use records from Antarctic whalers, who recorded the location of every whale kill. Because whales tend to feed along the ice edge, these records can be used to estimate ice extent at the time of hunting. Scientists also examine ice cores for signs of phytoplankton, since their presence in a core similarly reflects the sea ice boundary.
Meier, W.N. et al. Sea Ice. Arctic Report Card. NOAA Arctic Program. doi: 10.25923/y2wd-fn85.
National Snow & Ice Data Center. Arctic Sea Ice News & Analysis. https://nsidc.org/arcticseaicenews/ March 22, 2022.
NASA. Sea Ice. https://earthobservatory.nasa.gov/features/SeaIce
NOAA. How does sea ice affect global climate? https://oceanservice.noaa.gov/facts/sea-ice-climate.html
News & Insights
As Arctic Inuit communities try to cope with extreme food insecurity, researchers look for answers
The largest Arctic science expedition in history has ended, with the return of the German icebreaker Polarstern to its home port of Bremerhaven more than one year after it departed Tromso, Norway.
As glaciers melt at unprecedented rates, WHOI’s Simon Pendleton is looking back to historical records to predict whether this new cool runoff will slow ocean circulation and cool the northern hemisphere––findings which could mean adjustments to some climate predictions.
WHOI biologist Carin Ashjian is aboard the icebreaker Polarstern in the Arctic as part of the year-long MOSAiC research expedition. She should be almost home by now. Instead, her stay has been extended by COVID19.
WHOI scientist dusts off historical accounts to tackle the long-standing mystery of just how thick Arctic sea ice was in the early 19th century.
WHOI in the News
From Oceanus Magazine
Explore the beauty and science behind Greenland’s glaciers through this images from photographer Roger Fishman
A behind-the-scenes look at the making of the Emmy-nominated PBS film, Life in The Dark: The Polar Night
WHOI experts dig into a popular misconception that the Arctic is always frigid.
If ancient Beringians got to the Americas by boat, it couldn’t have been easy
Researchers challenge their own assumptions to improve sea-level rise predictions